Cycloinverter



April J. L. BOYER ET AL 2,707,258

CYCLOINVERTER Filed Aug. 30, 1952 5 Sheets-Sheet l qua men-

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ATTORNEY J, L. BOYER ET AL April 26, 1955 CYCLOINVERTER 5 Sheets-Sheet 2 Filed Aug. 30, 1952 60 Cycles 25 Cycles Fig.2.

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April 26, 1955 J, 1.. BOYER El AL CYCLOINVERTER 5 Sheets-Sheet 3 Filed Aug. 30, 1952 \NVENTORS John L. Boyer a Charles G. Hugensick. BY 9 ATTORNEY April 1955 J. L. BOYER ET AL 2,707,258

CYCLOINVERTER Filed Aug. 30, 1952 Fig.5. 3 5

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ATTORNEY April 26, 1955 J. BOYER El AL CYCLOINVERTER 5 Shets-Sheet 5 Filed Aug. 30, 1952 INVENTORS John L. Boyer 8. Charles G. Hugensick. BY

ATTORNEY United States Patent 2,707,258 CYCLOINVERTER Application August 30, 1952, Serial No. 307,294 19 Claims. (Cl. 318-197) Our invention relates to electronic power-converters consisting of a number of specially controlled electronic tubes connected between two alternating-current circuits. The converters to which our invention applies are singleconversion power-converters, by which we mean that there is a single stage in the power-transfer, or a direct transfer of power from one alternating-current circuit to the other alternating-current circuit, without passing through any intermediate power-form such as direct-current power.

In previously known electronic power-converters, the tubes have been controlled by excitation-equipment which is related to the frequencies of both the power-supplying and power-receiving alternating-current circuits, as shown for example in our Patent No. 2,442,262, granted May 25, 1948, on which our present invention is an improvement.

Our present invention makes use of special tube-controlling means, whereby the tubes are controlled only in response to the alternating-current system which is receiving power, without any tube-control which is depend out upon the supply-system. The receiving-system must therefore be one which has an established voltage and is able to supply the reactive volt-amperes which are required for the commutation of the tubes.

In order to be operative, the tubes must be released, or put in a condition in readiness to become conducting when the tube-anode becomes positive with respect to the tube-cathode, at an inversion-angle relative to the phaseterminal of the receiving circuit which is connected to any particular tube, from which circumstance we have applied the term cycloinverter to our new conversionapparatus, to distinguish it from the previously known cycloconverters in which the tubes were released or fired at both rectifier-angles and inverter-angles. Thus, in our cycloinverter, there are no control-elements related to the power-supplying system, and the tubes are never released at rectification-angles, as in the cycloconverter. By this method of control, there are available, at all times,

counter-voltages which oppose the conduction of either positive or negative currents. Actual conduction then occurs when the instantaneous impressed voltage exceeds the counter-voltage of the cycloinverter. This is the simplest form of control which has yet been devised for the electronic conversion of power directly from one alternating-current system to another.

With the foregoing and other objects in view, our invention consists in the circuits, systems, apparatus, combinations, parts, and methods of design and operation, hereinafter described and explained, and illustrated in the accompanying drawing, wherein Figure 1 is a diagram of circuits and apparatus involving an illustrative form of our invention as applied to the control of the speed of a wound-rotor inductionmotor,

Fig. 2 is a similar View of a cycloinverter-circuit for transferring power in either direction between a 60-cycle power-system and a 25-cycle power system, both powersystems having an established voltage and being able to supply the required reactive kva for commutation when it is receiving power from the other system,

Fig. 3 is a similar view showing an illustrative arrange ment of the cycloinverter, in which an established powersystem is used as the receiving-system for receiving power from a variable-speed generator, which can be driven so as to have a variable frequency which is either above or below the frequency of the established power-system.

Fig. 4 is a view of a cycloinverter-system similar to Fig. 3, except that the cycloinverter-tubes are connected and controlled for single-phase operation with respect to the cycloinverter-tubes, instead of polyphase operation,

2,707,258 Patented Apr. 26, 1955 k! on the receiver-side, a diametrical six-phase connection being shown, by way of example, and

Figs. 5, 6, and 7 are wave-form diagrams which will be referred to in the explanation of the operation of our invention.

In the illustrated forms of embodiment of our invention, we have shown the cycloinverter-elements as consisting of ignitrons 10, which are electronic elements or tubes, each of which is controlled as an inverter-tube with respect to the power-receiving system, as will be subsequently described. Each of the tubes 10 has a single main anode a, a mercury pool type of cathode c, a make-alive electrode or ignitor i, usually also an auxiliary or holdinganode h, and sometimes also, (as shown in Fig. 2), a grid g. We wish it to be understood, however, that these ignitrons 10 are to be regarded as being broadly illustrative or representative of any unidirectionally conducting cycloinverter-elements, each unidirectionally conducting element having a main anode-and-cathode circuit and a control-circuit of such types that the main anode-andcathode circuit does not become conducting unless the control-circuit is in a released condition at the time when the main anode is positive with respect to the main cathode, in any particular element.

In Fig. l, we show a cycloinverter consisting of eighteen tubes 10, and illustrative control-circuits therefor. The cycloinverter-tubes 10, in Fig. 1, are shown as a powerconversion means for feeding three-phase power, from the secondary SEC of a wound-rotor induction-motor M, back to an established three-phase power-system, which also supplies power to the primary winding of the motor M. The phase-terminals of the motor-secondary SEC, which constitutes the supply-system for our cycloinverter in Fig. 1, are indicated by the letters A, B, and C, while the phase-conductors of the established power-line or system, which receives power from the cycloinverter in Fig. l, are designated 1-4, 36, and 5-2, respectively, being numbered as for a six-phase system, in accordance with a known convention.

in all of the illustrated forms of embodiment of our invention, the cycloinverter-elements or tubes 10 are given individual reference-characters consisting first of a nu meral, from 1 to 6, identifying that tube with the receiving-circuit terminal to which it is connected, followed by a distinguishing letter. In Figs. 1, 2, and 3, the distinguishing letters which constitute a part of the referencecharacters for the several tubes 10 are the letters A, B, and C which correspond to the designations of the supplycircuits A, B, and C. In Fig. 4, the distinguishing letters are P and N, respectively, to indicate that the invertertubes are positively or negatively connected with respect to the receiving system 1 to 6, as the case may be.

The various cycloinverter-tubes it; may be regarded as being grouped into a plurality of groups, each group having a number of tubes or cycloinverter-elements, as required by the number of terminals of the receiving circuit to which energy is being supplied by that group. In Figs. 1, 2, and 3, each group of tubes supplies energy to each phase of a three-phase receiving circuit, so that there are three tubes in each group. Half of the total number of groups of tubes have a common supply-side cathode-terminal, which is common to the tubes of its own group, as indicated at Ac, Bc, and Cc, in Fig. 1. Each of the remaining groups has a common supplyside anode-terminal, as indicated at Aa, Ba, and Ca, in Fig. 1.

Each of the phase-terminals A, B, and C of the circuit which supplies energy to the cycloinverter is connected to the correspondingly lettered cathode and anode-terminals of its two groups of tubes. In many instances, as will be subsequently described, and as is shown in Fig. 1, these circuit-connections include paralleling reactors XA, XB and XC, respectively. Thus, the midpoint of the paralleling reactor XA is connected to the supply-circuit terminal A, and the terminals of the paralleling reactor XA are connected respectively to the common cathodeterminal Ac of the group comprising the tubes 1A, 3A and 5A, and to the common anode-terminal Aa of the group which comprises the tubes 4A, \EA and 2A' In this system of reference-characters for designating the tubes in Fig. l, the circuit which directly receives power from the cycloinverter-tubes is a three-phase circuit, having terminals which are designated 1, 3, and 5, respectively. The tubes which are positively connected, as inverter-tubes, to the receiving system 1, :5, 5, are designated by the numerals 1, 3, and a, to indicate that the anodes of these inverter-tubes are connected to the designated phase-terminals of the receiving c1rcuit. On the other hand, the inverter-tubes Which are negatively connected to the receiving system 1, 3, 5, or which have their cathodes connected to the several receiving-circuit terminals 1, 3, and 5, are deslgnated by the numerals 4, 6, and 2, respectively.

In the form of our invention which is shown 1n F g. 1, a variable-voltage power-transformer 11 is used, either as an instrumentality for controlling the speed of the motor M (or assisting in such speed-control), or as an instrurnentality for enabling the designer to use dlfferent voltages for the receiving circuit 1, 3, 5, which receives power from the cycloinverter, and the power-line 1-4, 3-6, 52, which receives feed-back power from the receiving circuit 1, 3, 5 of the cyclomvert er. Such a power-transformer 11 might be connected in either the primary circuits or the secondary circuits of the motor M (provided that the circuit-frequency does not approach zero), and if it is connected in the secondary Cll'ClllLS of the motor, it could be connected either on the powersunplying side or the power-receiving side of the cycloinverter. As shown in Fig. l, the power-transformer is connected on the power-receiving side of the CYCiOlIlverter, so that it is connected between the circuit 1, 3, 5, and the power-line 1-4, 3-6, 5 -2.

The cycloinverter equipment which shown in Fig. 1 also includes inverter-operation control-means, for controlling the tubes 10 from the alternating-current c1rcu1t 1, 3, 5, which receives energy from the cyclo-mvertertubes 10. In Fig. 1, since the receiving circuit 1, 3, 5 feeds power back to the main power-line 1fl, 3-6, 5-2, the receiving circuit of the cyclo1nverter 1s essentially the same as the power-circuit which supplies power to the primary of the motor M, whereas the secondary of the motor constitutes the power-supply system A, B, C for the cycloinverter.

The control-circuit power or energy, for the cycloinverter-tubes in Fig. 1, is supplied from an auxiliary transformer 12, the primary of which is connected to the main power-line. In almost every application of our invention, it is desirable to be able to control the inverter phase-angle at which the respective tubes are controlled, as such angle-control is readily provided, and

constitutes a useful means for controlling the average or effective back-voltage of the converter. In Fig. l, we have shown a typical control-angle controlling-means, in the form of a phase shifter 13 which is connected between the auxiliary transformer 12 and the polyphase control-terminals p1, 3, 5, which are used in the control of the several tubes 10. This control-circuit 1, 3, 5 is shown as energizing an ignitor-feeding transformer 1 and three holdingcircuit transformers 15, 16, and 17.

The ignitor-feeding transformer 14 is shown as havmg three secondary phase-terminals S1, S3, and S5, and a secondary neutral terminal SN, which are used to energize any suitable type of firing-circuits 18 for the gnitors i of the various tubes. The firing-circuits 18 wh1ch are shown in Fig. l are a known reactor-type of firing-circuit, which supplies peaked short-duration firing-lmpulsesto the various ignitor-circuits, and no detailed description of these firing-circuits 18 is believed to be necessary. The ignitor-circuits of the various tubes are indicated by the tube-designation, such as 1A, followed by the suffix i, to indicate the ignitor-circuit of that tube.

It is essential, in accordance with our present 1nven tion (using inverter-tubes which are exclusively controlled from the receiving circuit), that the firing-circuits 18 shall be excited at inversionangles with respect to the receiving circuit of the cycloinverter.

While we have described the ignitor-exciting circuits as firing circuits, we do so only because we show a type of circuit which is usually known as a firing circuit. Actually, of course, no tube fires, in the sense of becoming conductive, until its anode is more positive than its cathode. We prefer, therefore, to speak of the control-operation of releasing the tubes, rather than firing the tubes, at a desired inversion-angle, using the term releasing to apply to the condition in which the tube stands ready to commence conducting as soon as its anode becomes more positive than its cathode. In the broadest aspects of our invention, we desire that the tube-releasing ignitor-firing circuits shall be regarded as being illustrative or representative of any kind of tubecontrolling means which will bring about the tube-releasing operation as just discussed.

In all forms of our invention, each supply-circuit terminal, such as A, is connected to two groups of tubes, one of which is conducting when that supply-circuit terminal is positive, as for example the tubes 4A, 6A, and 2A in Fig. 1, while the other group of tubes is conducting when that supply-circuit terminal A is negative, as is the case for the tubes 1A, 3A, and 5A in Fig. 1. In each case, each group of tubes is connected, on its other side, to a plurality of receiver-circuit terminals, and all of the tubes are controlled at inversion-angles from their respective receiver-circuit terminals.

Since we use different groups of positively and negatively conducting tubes, a complete circuit for the anodecathode circuit of any tube which is of a positive polarity with respect to the supply circuit, (or of negative polarity with respect to the receiving circuit), will have to flow through a serially connected tube of opposite polarity, before the circuit for the current can find a return-path back to the supply-circuit. Since, however, our tubes are fired or released according to the voltages of the several receiving-circuit terminals, it generally happens that the firing-time of one of the two serially connected tubes, in any complete current-transmitting circuit through the cycloinverter, will be delayed by a certain time or phaseangle after the firing-time or instant of the other serially connected tube.

Consequently, it is, in general, necessary, at least when the cycloinverter is first started, and generally throughout the subsequent operation of the apparatus, that the releasing energizations which are effected by our controlcircuits, shall be sustained energizations, which hold the respective tubes in readiness to become conducting, over a considerable portion of a receiving-circuit cycle. In the broader aspects of our invention, any sustained-effect releasing-circuits may be used, whether involving suitably energized ignitors alone, or the combined use of impulseexcited ignitors and sustained-excited holding-anodes, or grids, in certain types of tubes, or other means for accomplishing the stated result.

In the particular form of our invention which is shown in Fig. 1, our tubes 10 are ignitrons, and the releasingcontrol is effected by ignitors or starting-electrodes. While it is known that ignitors or starting electrodes can be so excited as to have a sustained excitation during a considerable portion of a cycle, we prefer to use the impulse-type of firing-circuit, and to obtain our releasesustaining effect by the use of auxiliary or holding-anodes h, which we excite from one of the several illustrated holding-circuit transformers 15, 16, and 17, in Fig. 1. In this way, the holding-circuit of each tube, which is designated by the tube-desgnation, such as 1A, followed by the suffix h to indicate the holding-anode circuit, is energized with a sustaining voltage which usually begins before the firing-point, or instant of energization of the ignitor-circuit, such as the circuit 1A1, for tube 1A. The holding circuit does not become effective until the releasing-time of the ignitor-circuit, at which time an arc is formed, and held, by the holding-anode h. The sustaining time, during which the holding-circuit holds its are, after the firing or releasing of the ignitor i of that tube, may be for any number of receiving-circuit degrees, such as 60 or by way of example, which may be required by the particular exigencies of the particular cycloinverter circuit-connections which are used.

In Fig. 1, where the power-transfer is from a lowfrequency system to a high-frequency system, a 60 sustained-release time would be acceptable, measured in receiving-circuit degrees, but it would be practical also, if desired, to use a 120 holding-circuit time, after the instant of initial firing or release as shown in Fig. 1.

In Fig. 1, for example, the ignition or tube-releasing angle of each tube, for example the tube 1A, lags approximately behind the ignitor-firing voltage, such as S1, which is supplied by the transformer 14. The holding-circuit 1Ah of this tube is energized from a secondary voltage of the holding-circuit transformer 15, which lags 120 behind the firing-transformer voltage S1, thus causing the application of a positive holding-circuit voltage to occur (165-420) or 45 ahead of the actual ignition or releasing time of the tube, and the holdingcircuit continues to have a positive voltage for (18045) or 135 after the instant of firing or releasing the tube, thus holding the tube in readiness to conduct for this portion of the receiving-circuit cycle. In accordance with the usual practice, each holding-circuit includes a current-limiting resistance R, and a dry-plate-type or contact-rectifier 19.

Our cycloinverter is adapted to receive alternating current power, either single phase or polyphase, and at any frequency within the time-constant or deionizing capabilities of the tubes, and to deliver alternating-current power directly to a receiving-system, which may be either single phase or polyphase, and which may have a frequency either higher or lower than the supply-circuit frequency. The necessary conditions are that the receiving circuit shall have an established voltage, that is, a voltage and frequency of its own, either constant or variable, and that it shall be able to supply the inductive 1y reactive wattless currents which are necessary to commutate the currents from one tube to another.

In Fig. 2, for example, we have shown a cycloinverter circuit for transferring power in either direction between a 60-cycle power-system A, B, C, and a -cycle powersystem 14, 36, 52. The cycloinverter-circuit of Fig. 2, is somewhat similar to that of Fig. 1, except that the control-circuits of the cycloinverter of Fig. 2 include a plurality of double-throw switches 21, 22, and 23, which are mechanically connected together, as indicated by dotted lines 24, for the purpose of choosing whether the power-transfer is to be from the -cycle system to the 25-cycle system or vice versa, the point being that the control-circuits have to be taken from whichever system is receiving power.

In more detail, the cycloinverter-circuits of Fig. 2 may be described as follows. The 60-cycle power-line or system A, B, C is connected, through a power-transformer T60, and serially connected current-limiting reactors X60, to a 60-cycle cycloinverter circuit gbA, B, C, which may be either the supply-circuit or the receiving circuit of the cycloinverter. In like manner, the 25-cycle power-line or system 1-4, 3-6, 5-2, is connected, through a power-transformer T25, and serially connected current-limiting reactors X25, to a 25-cycle cycloinverter-circuit 1, (p3, 5, which may be either the receiving circuit or the supply-circuit for the cycloinverter.

The cycloinverter tubes 14), in Fig. 2, are given designations including numbers and letters corresponding to their phase-connections to the cycloinverter circuits (p1,

3, 5, on the one hand, and A, B, C on the other hand. It will be noted that each of the tubes 10, in Fig. 2, is directly connected to its associated supplycircuit conductor and receiving-circuit conductor, without the intermediary of the paralleling reactors, such as were associated with the supply-circuit conductors, as shown in Fig. 1 at XA, XB, and XC, respectively. This means that the cycloinverter of Fig. 2 must be used with only a limited amount of advanced firing-angle or inversion-angle, so as not to short-circuit tubes havmg opposite polarity, as will be subsequently explained. If a broader firing-angle phase-control were desired, In Fig. 2, it would be necessary to provide somewhat complicated switching-circuits, so that the paralleling reactors would be used only in the connections between the cycloinverter-tubes and the phase-terminals of the supplycircuit, according to whichever one of the two power-lines is supplying power to the other.

Each of the tubes 10, in Fig. 2, is also additionally shown as being provided with a grid g, which is a part of a grid-circuit such as 4Ag.

The control-circuits for the cycloinverter of Flg. 2 are provided by 60-cycle and 25-cycle auxiharyftransformers A60 and A25, respectively. Corresponding to each of the eighteen tubes 10, there is a separate tubecontrolling transformer 4At, 482., etc., these transformers being energized from either the 25-cycle or 60-cycle auxiliary transformers A25 or A60, under the control of the switch 21, according to which power-circuit 1s receiving power from the other power-circuit.

The secondary circuits for the tube-transformer 4A! are shown in detail, by way of example, in Fig. 2. The secondary circuit of this transformer 4At is used to energize the 4A holding-circuit 4Ah, through a rectifier 19, a coupling-resistance R2 and a current-l1m1ting resistance til) R. The secondary circuit of the transformer 4A2 also supplies energy to a grid-bias transformer 4Ab, which energizes a grid-bias resistor 25 through a rectifier-bridge 26 and a ripple-smoothing reactor 27. The grid-bias resistor 25 is also shown as being shunted by a ripplesmoothing capacitor 28, in accordance with a known practice. The grid-circuit 4Ag, for example, serially includes the grid-bias resistor 25, a grid-resistor Rg, and the secondary winding of a grid-coupling transformer 29, the primary of which is energized across the grid-coupling resistor R2 in the corresponding holding-circuit 4Ah. in this way, the grid-coupling transformer releases the control-grid 3 after the excitation-anode It begins conducting, in a known procedure for high-voltage ignitrons.

The various ignitor-circuits 4A1, 481, etc., of the various tubes 10 in Fig. 2 are energized through various firing-circuits 18 from either the secondary circuits S25 of a 25-cycle ignitor-fecding transformer 14-25, or the secondary circuits $60 of a 60-cycle ignitor-feeding transformer 14-64 under the control of the selector-switch 23, only ten of the eighteen ignitor-circuits being shown in Fig. 2. In Fig. 2, the firing-circuits 18' differ from those shown in Fig. l by including saturabie phase-shift reactors 25 and poi for th 25 and 60'cycle firing-circuits respectively, instead of using the phase-shifter 13 of Fig. l. The ignition-angles are controlled, in Fig. 2, by varying the direct-current excitation of the phaseshift reactors 25 and 60, in a known manner.

In Fig. 2, the selector-switch 22 is used to determine which of the ignitor-firing transformers 1425 or 14-69 shall be excited from the secondary circuit of the 25- cycle auxiliary transformer A25, or from the secondary circuit of the 60-cycle auxiliary transformer A66, as the case may be.

Fig. 3 shows an application of our invention in which the cycloinverter is used to supply power to an established power to an established power-system 1-4, 36, 52, from the outphases A, B, and C of a generator G which may be driven at a speed, which may or may not be variable, so as to cause the generator G to have a frequency which may be above or below that of the established receiver-system 1-4, 36, 52. The generator may, or may not, be a synchronous generator. It is shown, by way of example, as a synchronous generator which is excited with direct-currents which are applied to a fieldwinding FLD. The control-connections of Fig. 3 use the same auxiliary transformer 12 and the same phase-shifter 13 as in Fig. 1. Only one phase of the ignitor-feeding transformer is shown, at 34 in Fig. 3, and one phase of the holdingcircuit transformer 35. It will be understood that the other control-circuit connections may be the same as were shown in Fig. 1.

Fig. 4 shows a variation in the cycloinverter circuits of Fig. 3, whereby, in Fig. 4, only twelve cycloinvertertubes It) are used, arranged in six groups, so that each of the supply-circuit phases, A, B, and C, is connected to two groups, one connected for positive current-conduc tion w1th respect to the supply-circuit, as indicated at A0, Ba, Cu, and the other group connected for negative current-conduction with respect to the supply-circuit p ases, as indicated at A0, B0 and Cc, respectively. In Fig. 4, each group of tubes consists of two tubes, which are connected to diametrically opposite phase-terminals or a six-phase receiving system having its terminals numbered from 1 to 6. The diametrically opposite receivercr'fcurtphases are thus 1, 4; 3, 6; and 5, 2; and the twelve tubes in Fig. 4 are numbered accordingly, with suffixes P and N added, to indicate whether the tubes are connected positively or negatively with respect to the receiving circuit, P indicating that the anode a of the tube is connected to the correspondingly numbered conductor of the receiving circuit, while N indicates that the cathode c of the tube is connected to the correspondingly numbered conductor of the receiving circuit. The four tubes carrying the numbers 1 and 4 are connected to the first supply-circuit phase A, while the four tubes carrying the numbers 3 and 6 are connected to the second supply-circuit phase B, and the four tubes carrying the numbers 5 and 2 are connected to the third supply-circuit phase C.

In Fig. 4, the receiving circuit 1 to 6 of the cycloinverter is connected, through a star-delta power-transformer 41 to an established three-phase power-circuit 1-4, 3-6, 52.

In Fig. 4, the various ignitor-circuits, such as lPi, and holding-circuits, such as 1Ph, are energized from the receiving circuit 14, 3-6, 52, by means of an auxiliar; transformer 42, and a phase'shifter 43, through an ignitor-feeding transformer 44, and two holding-circuit transformers 45F and 45N, respectively, in a manner similar to the control-system of Fig. 1, so that no further explanation is believed to be necessary, other than to note that the control-circuit phasing or timing is different, in Fig. 4, because of the difference between the single-phase or diametrically connected inverter-tube operation of Fig. 4, and the three-phase tube-operation of Fig. l, as will be evident from the subsequently described curve-diagrams in Fig. 6.

The operation of our invention will best be understood by reference to the somewhat idealized wave-form digarams which are presented in Figs. 5, 6, and 7.

Fig. 5 shows the wave-forms of the cycloinverter-circuits of Figs. 1 and 3, when the power-receiving system is of a higher frequency than that of the power-supplying system. Only one phase of the voltage impressed by the low-frequency supply-system is shown, namely, the line A-to-neutral voltage which is marked A in Fig. 5. The voltages of the high-frequency receiving system are indicated at 1., 3, and 5. The resultant counter-voltage of the cycloinverter, which opposes the phase A impressed vlotage, is indicated by heavy lines 51. The phase A supply-current is indicated at 52, segregated into the current-components which are supplied by the various tubes as designated in Fig. 5. The various ignitor-voltages for the receiver-phase 1-4 are indicated at 1A1, 1131', etc.

The curves of Fig. 5 are drawn for a condition in which all of the tubes are released, as at 53 on the receiver-wave 1, at an inverting ignition-angle a of about 140 delay after the positive crossing-point 54 between the next leading receiver-wave 5 and the receiver-wave 1 (for example) of the tube which is being fired. This is equivalent to an advance-angle it of 40 prior to the negative crossing-point 55 of the aforesaid two receiver-waves, such as S and 1. It must be understood, however, that the tubes conduct current only during the times when their anodes are more positive than their cathodes.

It is very difiicult to exactly pin down these tubeconducting periods with reference to the voltage-waves which are shown in Fig. 5, because of the various reactance-voltages of the system, which have a voltage-smoothing effect on the chopped-up voltages which are involved. In general, it may be said that the tubes conduct current only when the half-cycle average impressed supply-circuit sine-wave voltage, such as A, exceeds the average countervoltage E's of the resultant counter-voltage 51 for that phase, averaged over 180. It is to be noted that the phase A supply-currents 52 which are plotted are the currents which are supplied by the supply-circuit voltage A. The current which is positive with respect to the supply-circuit becomes negative with respect to the receiving-circuit. In Fig. 5, a certain commutating-period is shown, as indicated at 56, for assumed values of loadcurrent and commutating reactance. It is also to be understood that, in addition to the momentary ignitorimpulses 1A1, 4Ai, etc., the tubes are impressed with various holding-circuit voltages which have been de scribed in connection with Figs. 1 and 3, and which are not shown in the wave-forms of Fig. 5.

Considering the operation of the motor-controlling system of Fig. 1, with respect to the curve-diagrams of Fig. 5, it will be noted that the ignition angle 5 may be varied from a suitable minimum advance-angle value (which must be large enough to admit of commutation, plus a margin-angle for tube-deionization) to a maximum advance-angle of 90 (or 90 plus the commutating angle), and during this time the speed of the motor M can be varied from zero to substantially its full synchronous speed. Secondary power, at a variable frequency, is thus returned to the power-line (which is the receivingcircuit of the cycloinverter), by inverter-action of the tubes. The motor-speed, under a given load, is determined by the ignition-angles a or ,8 which, in turn, determine the cycloinverter countervoltage 51 or E'd which opposes the flow of current from the motor-secondary SEC.

The motor will operate at its maximum speed in Figs. 1 and 5, when the advance-angle [i is such as to result in a zero countervoltage at whatever current is being carried by the cycloinverter. This value of the advanceangle 3 is 90 at no-load, which is also a 90 delay-angle a as a rectifier. Under load-conditions, the advanceangle [3, which produces a zero countervoltage in the cycloinverter, is somewhat greater than 90". Under these conditions, the average voltage Ea of the tubes will be Zero, but there will be instantaneous voltages 51, in both positive and negative directions which will force wattless currents to How through the paralleling reactors XA, XB, and XC. At the same time, there will be a zero power-factor loading on the power-line 14, 3-6, 52, which is the receiving circuit of the cycloinverter.

This zero-power-factor operation can be avoided, if the motor M is designed so that its required or rated full speed will occur at lower advance-angles B, preferable or less, so that the motor will be operating at a predetermined slip at its highest actual operating speed. Under these conditions, the paralleling reactors XA, X8, and XC will not be necessary in Fig. 1, because there will be no danger of short-circuiting a phase of the receiving system through two serially connected tubes, since the instantaneous voltage of the tube 1A can never be more positive than that of the tube 4A, etc.

It is also to be noted, in Fig. 1, that the power-transformer 11 may be used, either with or without voltagechanging equipment, and when this transformer is used, the motor-secondary SEC may be designed so that, at zero speed, the secondary voltage may have any convenient value, usually less than the primary voltage of the motor.

Also, if the power-transformer 11 has voltage-varying taps, these taps may be adjusted so as to continue to somewhat reduce the secondary voltage of the motor (thus continuing to increase the speed of the motor), to a value greater than zero voltage, after the advance firing-angle B has been increased to a maximum of 60 or less, thus avoiding the necessity for the paralleling reactors XA, XB, and XC, while at the same time being able to bring the motor up to higher speeds, at which there is an acceptable value of slip, also obtaining a better overall power factor. A variable-voltage power-transformer 11 thus serves as a means, separate from the cycloinverter-tubes and their control, for varying the ratio of the voltages of the primary and secondary voltages of the motor, or of the power-receiving and power-supplying circuits of the cycloinverter. Or, considering the motor-secondary SEC as a generator, a variable-voltage power-transformer, such as 11, constitutes a means, separate from the cycloinverter, for varying the ratio between the generator-voltage and the generator-speed.

It is possible, in Fig. l, to use the power-transformer 11 with voltage-varying taps, in cooperation with the paralleling reactors XA, XB, and XC, and a complete range of phase-control so as to be able to vary the cycloinverter countervoltage from a maximum value to zero. In this way, operation at high motor-speeds can be obtained by using the low transformer taps and large advance-angles, so as to reduce the kva. taken from the supply system, thereby improving the overall power factor. It is advantageous to simultaneously reduce the transformer-taps and to increase the advance'angle 8, to increase the speed of the motor, or to enable the motor to carry a higher load at a constant speed.

Considering, now, the operation of the variable-speed generator-system of Fig. 3, with reference to the curvediagrarns of Fig. 5, it will be noted that the power transferred between the generator G and the established receiving system 14, 36, 5--2 is dependent upon the relation between the generator-voltage (such as the phase A voltage A in Fig. 5) and the average counter-voltage Ea of the cycloinverter. The value of the generatorvoltage, at any given generator-speed, may be adjusted by field-control; or, if the field-control is left fixed, and the generator is driven by a variable-speed prime-mover (not shown), the generator-voltage will vary according to the speed of the generator and its prime-mover, and that speed will vary, of course, with the amount of power-input into the prime-mover. On the other hand, the average countervoltage E'd of the cycloinverter depends upon the actual jagged-line counter-voltage 51 of Fig. 5, which in turn depends upon the ignition-angle of the tubes.

The load-current which is supplied from the generator G to the receiving circuit, in Fig. 3, is determined by the differences in the averages of the generator-voltage and the counter-voltage of the cycloinverter. Instantaneous differences in the impressed and countervoltages will be across the reactances of the generator G and the receiving circuit. Usually, in variable-speed generator-systems using our cycloinverter, as in Fig. 3, it is not necessary to use advance-angles B of more than 60, in which case there will be no circulating currents between the two tube-groups, such as the Ac group and the Aa group, which are connected to a given generator-phase, such as A; and hence there will be no necessity for the paralleling reactances XA, XB and XC which are shown in Fig. 3. In the general case, however, without any limitations as to the value of the advance ignition-angle ,3, the paralleling reactances XA, XB and XC will be needed, in order to permit the use of the higher advance-angles, as shown in Fig. 3.

Fig. 6 shOWs the (somewhat idealized) wave-forms for the variable-generator system of Fig. 4, which uses a diametrical six-phase inverter-tube connection, as distinguished from the three-phase inverter-tube connection of Fig. 3. The diametrical six-phase receiving-circuit connections, in Figs. 4 and 6, are really three singlephase inverter-tube connections, with each supply-phase, or generator-phase, such as A, connected to two groups of tubes, one group for carrying the positive half-cycles of the generator-current, while the other group carries the negative half-cycles of the generator-current in that phase.

in Fig. 6, one phase of the generator A-to-neutral voltage is shown at A, while the six phases of the receivingcircuit voltage are shown at 1 to 6. The cycloinverter counter-voltage, which opposes the phase-A generator supply-circuit voltage A, is indicated in Fig. 6 by heavy lines 61. The ignitor-circuit voltages are shown at 1N1", 4Pi, 1P1, and 4Ni in Fig. 6, for the tubes which are connected to the diametrical receiving-circuit phase 1-4.

In the diametrical six-phase cycloinverter-system of Fig. 4, it is necessary to use the paralleling reactors XA, XB and XC under all operating-conditions, because, as shown in Fig. 6, the positively and negatively conducting tubes, such as 1? and 4N, are released at the same instants, rather than 60 apart, as in Figs. 1, 2, and 3. The reactor-voltage, for instance the voltage across the reactor XA, appears whenever two oppositely connected tubes (such as IP and 4N) are simultaneously released, and this reactorvoltage becomes larger, as the inverter advance-angle [3 is increased. Typical reactor-voltages are indicated by the shaded portions 62 in Fig. 6.

In Fig. 4, the excitationarcs or holding-circuit voltages must be maintained for at least 60 of the receiving system, after the firing or releasing of the ignitor-circuits, in order to release enough tubes for a complete circuit, since the diametrically connected inverter-tubes, which are connected to different reactors XA, KB and XC, are released at different instances. In the particular circuit which is shown in Fig. 4, the excitation-arcs are main tained, on the holding-anodes h, for considerably more than 60 of the receiving system, which is quite permissible.

Fig. 7 shows the cycloinverter wave-forms for the conversion-circuit of Fig. 2, when the power-receiving system (such as the 25-cycle system) is of lower frequency than the power-supplying system (such as the 60-cycle system), corresponding to the illustrated lefthand positions of the changeover-switches 21, 22, and 23 in Fig. 2. In Fig. 7, the 60-cycle supply-line voltages are indicated at A, B, and C, while the 25-cycle receivingcircuit voltages are indicated at 1, 3 and 5. The ignitorcircuit impulses are indicated in Fig. 7 at 3A1, 381', etc. The holding-circuit voltages are not shown in Fig. 7. Superimposed on the supply-circuit voltages A, B, and C, in Fig. 7, is also a heavy-line curve 71, indicating the resultant counter-voltage of the cycloinverter, for the receiving circuit phase 1-4. Fig. 7 also shows the tube-circuit currents 72 for the receiving-circuit phase 1-4, with subdivisions to show the currents carried by the several tubes.

In Fig. 7 it will be noted that the tubes 4A, 4B and 4C, which are negatively connected with respect to the receiving-circuit phase 1, are all released simultaneously at a point '73 on the receiver phase 1, which is seen to be at an advance-angle with respect to the negative crossingpoint 74 between the previously conducting next-leading phase and the phase 1 which we are considering. This releasing-time is shown, on the supply-circuit waves A, B

ative crossing-point 77 and C, at 73A, 73B, and 73C, respectively. At any finite current-value, there is a certain commutating-period 75, which delays the commencement of current-conduction, by the tube 4C, until the point 76 is reached, on the receiving phase 1, corresponding to the point 76C on the supplyphase C.

Current is commutated, in Fig. 7, from the supplyphase C to the next-following supply-phase A, at the negbetween these two supply-phases. At this time, as indicated at 77, the anode-voltage of the tube 4A begins to become more positive than the cathodevoltage of that tube, and a commutating period commences, as indicated at 78, until a point 79 is reached, at which the supply-current is completely transferred over from the supply-phase C to the supply-phase A. The anode of the tube 4B, which was released at 738, does not become positive until the crossing-point 81 is reached, and the current is not completely transferred from the tube 4A to the tube 48 until the point 83 is reached, as indicated on the supply-voltage curve B. Current can commutate between the 60-cycle supply-phases A, B, and C because the holding or excitation-anodes 11 hold the arcs for sufficient intervals after the releasing-points 73C, 73A, and 7313. The period of excitation of these holding-anodes, after the firing or releasing-points of the ignitors, is usually from about to on the 25- cycle receiving system.

The 60-cycle power-supply system A, B, C can supply current only when its average voltage Es, as shown in Fig. 7, is greater than the average counter-voltage E'd of the 25-cycle receiving circuit, at any given inverting-angle. The negative counter-voltages of the 25-cycle receiving circuit are indicated by the heavy-line curve 91, which is superimposed on the receiving-circuit Waves 1, 3, and 5 in Fig. 7.

In Fig. 2, the series reactors X60 and X25 cooperate with the reactances of the power-transformers T60 and T25 in absorbing instantaneous voltage-differences between the two power-lines, such as are shown in Fig. 7. The series reactors X60 and X25 may, or may not, be required, depending upon the peak currents and the fault currents to be allowed. The commutating reactances of the transformers T60 and T25 may be sutlicient, in some cases. It is desirable to make the total reactance of the power-receiving system a minimum, and for this reason, it may sometimes be desirable to provide reactor-shorting switches X860 and X825 around the reactors X60 and X25, respectively, in Fig. 2. When such switches are provided, the switch X825 may be closed, if desired, to short out the reactors X25 when the 25-cycle system is receiving power, and the switch X860 may be closed, if desired, to short out the reactors X60 when the 60-cycle system is receiving power.

The wave-forms of Fig. 7, with suitable changes in reference characters, can also be applied to the circuit of Fig. 3, when the generator G is operating at frequencies above that of the power-receiving system.

In like manner, the wave-forms of Fig. 5, with suitable changes in the reference characters, can also be applied to the circuit of Fig. 2, when the changeover-switches 21, 22, and 23 are moved to their right-hand positions, for the case in which power is being transmitted from the low-frequency (ZS-cycle) system to the higher-frequency (60-cycle) system.

While we have described our invention, and explained its principles of operation, with respect to only a few illustrative forms of embodiment, we wish it to be understood that our invention is not altogether limited to these particular forms of embodiment. We also wish it to be understood that we are not limited to various circuitdetails or elements of the described combinations, as various equivalent elements may be substituted, and also various changes of omission and addition may be made, without departing from the essential spirit of our invention.

We claim as our invention:

1. A cycloinverter for transferring power in one direction between two alternating-current circuits of diverse frequencies, at least the receiving circuit having an established voltage and being able to supply the reactive voltamperes required for commutation of the cycloinverterelements, said cycloinverter comprising: (a) a plurality of groups of unidirectionally conducting elements, each group having a number of elements as required by the number of terminals of the receiving circuit to which energy is being supplied by that group, half of the groups having a common cathode-terminal for each group, and each of the remaining groups having a common anode-terminal, each unidirectionally conducting element having a main anode-and-cathode circuit and a control-circuit of such types that the main anode-and-cathode circuit does not become conducting unless the control-circuit is in a released condition at a time when the main anode is positive with respect to the main cathode, (b) circuit-connections for connecting each phase-terminal of the supplycircuit to a common cathode-terminal and a com mon anode-terminal of. its two groups of elements, one group to conduct positive current and the other to conduct negative current, (c) circuit-connections for connecting the remaining terminals of the respective elements to their corresponding phase-terminals of the receiving circuit, ((1) an inverter-operation control-means, for applying a releasing-voltage to the control-circuits of all of the unidirectionally conducting elements which are connected to any terminal of the receiving circuit, said inverter-operation control-means comprising a means for deriving a timing control-circuit voltage solely from the receiving circuit, and a circuit-connection means for applying said timing control-circuit voltage to the respective control-circuits in such manner as to obtain a releasing-voltage only once during every receiving-circuit cycle at the same inversion-angle relative to that terminal of the receiving circuit, and (e) a load-controlling means for varying the relation between the average of the used portions of the voltage-waves of the higherfrequency circuit and the average voltage of the lowerfrequency circuit.

2. The invention as defined in claim 1, in combination with: (f) a paralleling reactor disposed between each supply-circuit terminal and the cathode and anode-terminals of its two groups of elements.

3. The invention as defined in claim 1, characterized by said load-controlling means (2) comprising a means for varying the inversion-angle at which the control-circuits of the unidirectionally conducting elements are released.

4. The invention as defined in claim 1, characterized by said load-controlling means (e) comprising a means, separate from the cycloinverter-elements and their control, for varying the ratio of the voltages of the two circuits 5. The invention as defined in claim 1, characterized by the releasingenergizations of the control-circuits being sustained energizations, holding the respective unidirectionally conducting elements in readiness to become conducting over a considerable portion of a receiving-circuit cycle.

6. The invention as defined in claim 1, characterized by the releasing-energizations of the control-circuits being brief impulses of short duration, each unidirectionally conducting element also having a holding-anode circuit, and a holding-circuit energization-means, for maintaining a positive voltage on each holding-anode circuit for a considerable portion of a receiving-circuit cycle after the releasing energization.

7. The invention as defined in claim 1, characterized by the supply-circuit having a higher frequency than the receiving circuit.

8. The invention as defined in claim 1, characterized by the supply-circuit having a lower-frequency than the receiving circuit.

9. The invention as defined in claim 1, characterized by said supply-circuit including a variable-speed dynamoelectric machine for generating, at the supply-circuit terminals, an alternating-current voltage having a magnitude which is dependent upon the speed of the machine.

10. The invention as defined in claim 9, characterized by the load-controlling means comprising means for varying the relation between the speed and the supply-circuit voltage of the machine at any given load.

11. A motor-control system for a wound-secondary induction-motor adapted to operate from the line-conductors of an alternating-current supply-system of substantially fixed frequency and voltage, said motor-control system comprising a cycloinverter for feeding power back from the motor-secondary to the line-conductors of the supplysystem, said cycloinverter comprising: (a) a plurality of groups of unidirectionally conducting elements, each group having a number of elements as required by the number of line-conductors of the supply-system, half of the groups having a common cathode-terminal for each group, and each of the remaining groups having a common anodeterminal, each unidirectionally conducting element having a main anode-and-cathode circuit and a control-circuit of such types that the main anode-and-cathode circuit does not become conducting unless the control-circuit is in a released condition at a time when the main anode is positive with respect to the main cathode, (b) circuit-connections for connecting each terminal of the motor-secondary to a common cathode-terminal and a common anode-terminal of its two groups of elements, one group to conduct positive current and the other to conduct negative current, (c) circuit-connections for connecting the remaining terminals of the respective elements to their corresponding line-conductors of the supply system, (:1) an inverteroperation control-means, for applying a releasing-voltage to the control-circuits of all of the unidirectionally conducting elements which are connected to any line-conductor of the supplysystem, said inverter-operation controlmeans comprising a means for deriving a timing controlcircuit voltage solely from the supply-system, and a circuitconnection means for applying said timing controlcircuit voltage to the respective control-circuits in such manner as to obtain a releasing-voltage only once during every supply-system cycle at the same inversion-angle relative to the phase of that line-conductor, and (e) motorcontrolling means for varying the relation between the average of the used portions of the voltage-waves of the supply-system and the average voltage of the motorsecondary.

12. The invention as defined in claim 11, in combination with: (f) a paralleling reactor connected between each motor-secondary terminal and the cathode and anode-terminals of its two groups of elements.

13. The invention as defined in claim 11, characterized by said motor-controlling means (e) comprising a means for varying the inversion-angle at which the control-cir- 1cuitsdof the unidirectionally conducting elements are reease 14. The invention as defined in claim 11, in combination with: (f) a transformer connected to the supply-system so that there is one set of line-conductors for the motor-primary and another set of line-conductors, of difierent voltage, for the motor-secondary.

15. The invention as defined in claim 14, characterized by the motor-controlling means of claim 11 (e) including a means for varying the inversion-angle at which the control-circuits of the unidirectionally conducting elements are released.

16. The invention as defined in claim 14, characterized by the motor-controlling means of claim 11 (e) including a means for varying the voltage-ratio of the transtormer or clalm 14 (I).

17. The invention as defined in claim 14, characterized by the motor-controlling means of claim 11 (e) including a means for varying the inversion-angle at which the control-circuits of the unidirectionally conducting elements are released, and also including a means for illzrgfiaig the voltage-ratio of the transformer of claim 18. The invention as defined in claim 11, characterized by the releasing energizations of the control-circuits being sustained energizations, holding the respective unidirectionally conducting elements in readiness to become conduclting over a considerable portion of a supply-system cyc e.

19. The invention as defined in claim 11, characterized by the releasing-energizations of the control-circuits being brief impulses of short duration, each unidirectionally conducting element also having a holding-anode circuit, and a holding-circuit energization-means, for maintaining a positive voltage on each holding-anode circuit for a considerable portion of a supply-circuit cycle after the releasing energization.

References Cited in the file of this patent UNITED STATES PATENTS 1,930,302 Bedford Oct. 10, 1933 2,077,206 Bedford Apr. 13, 1937 2,185,700 Bedford Jan. 2, 1940 2,213,945 Alexanderson Sept. 10, 1940 2,236,984 Alexanderson Apr. 1, 1941 2,264,854 Mittag Dec. 2, 1941 

